a) Field of the Invention
The present invention relates to an objective lens system using gradient index lens elements which have refractive indices varying dependently on distances as measured from an optical axis in a radial direction.
b) Description of the Prior Art
For sufficiently correcting aberrations such as spherical aberration, coma, curvature of field and chromatic aberration in an objective lens system, it is necessary to use a large number of lens elements, thereby leading to an increased manufacturing cost of a lens system.
As means for correcting this defect, it is known to use a gradient index lens elements which have larger freedom for correction of aberrations than homogeneous lens elements. As conventional examples of objective lens systems using gradient index lens elements, there are known objective lens systems disclosed by Japanese Patents Kokai Publication No. Sho 59-62816 and No. Sho 59-62817 each of which use a cemented lens component consisting of gradient index lens elements having refractive indices varying dependently on distances as measured from an optical axis in a radial direction and are cemented to each other. These conventional examples are configured as lens systems in which spherical aberration of monochromatic light and coma are corrected.
Further, a lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-124011 is known as a lens system in which chromatic aberration of white light is corrected.
Furthermore, known as examples of objective lens systems for microscopes using gradient index lens elements are a lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-31817 which is composed of three lens components and a lens system disclosed by Japanese Patent Kokai Publication No. Hei 6-3598 which is composed of two lens components.
Each of the above-mentioned Japanese Patents Kokai Publication No. Sho 59-62816 and No. Sho 59-62817 describes correction of aberrations of monochromatic light, but makes no reference to correction of chromatic aberration of white light. Japanese Patent Kokai Publication No. Sho 63-124011 describes correction of chromatic aberration of white light, but the lens system disclosed by this patent cannot correct chromatic aberration sufficiently as it has a large numerical aperture due to a fact that gradient index lens elements have small values of V.sub.10. These gradient index lens elements can hardly be manufactured in practice since they have axial and offaxial refractive indices which are largely different from each other.
Moreover, the objective lens system for microscopes disclosed by Japanese Patent Kokai Publication No. Sho 62-31817 is scarecely capable of correcting aberrations sufficiently when it has a large numerical aperture.
In addition, the objective lens system for microscopes disclosed by Japanese Patent Kokai Publication No.
Hei 6-3598 can hardly correct aberrations when it has a large numerical aperture. When an attempt is made to configure this objective lens system so as to have a large numerical aperture, gradient index lens elements used in a first lens unit thereof can hardly be manufactured in practice since they have axial and offaxial refractive indices which are largely different from each other.